The present invention relates to ball joints and is more particularly directed to molded plastic ball joints.
In a ball joint a ball moves within a socket so as to allow rotary motion in all directions within certain limits. Such joints are found in many consumer, commercial and industrial products where a range of movement is desired. By way of illustration, they are often used in occupancy detection devices of the sort that turn on a light in response to the presence of a person or other target object. A sensor head containing electronics for detecting the presence of the target object is connected to a mounting base through a ball joint that allows the sensor head to be aimed in a desired direction after the base is mounted in position.
In many products, and particularly mass produced products, the ball joint and product housing are often made of plastic. In one form of plastic ball joint, for example, a plastic socket or cup is formed having a generally hemispherical shape. A ball, positioned on the end of a support arm, is seated in the socket and is held there by several plastic fingers. The fingers extend forward from the margin of the hemispherical socket and are shaped to engage the ball and urge it into the socket. The fingers have sufficient give to permit the ball joint to be assembled by pressing the ball into the socket past the fingers, which deform slightly to permit the ball to snap into the socket and then have sufficient residual strength to retain the ball in the socket.
The problem with this arrangement is that the known ways for molding the plastic socket and fingers impose undesirable compromises. For example, to mold the socket and fingers as an integral unit, the mold may include a ball-shaped core to define the inner surfaces of the socket and fingers. When the mold is separated, the ball-shaped core is pulled out of the socket past the newly formed fingers (opposite to the manner in which a ball will later be urged into the socket when the ball joint is assembled). This action stresses the fingers. Thus when this method is used, all fingers are necessarily stressed at least twice (once to remove the core, a second time during assembly to install the ball). This tends to weaken the fingers, resulting in a greater failure rate and thereby lowering the effective yield of acceptable assembled ball joints. As a result, a greater number of fingers will be broken or found otherwise unacceptable, thereby lowering the effective yield of acceptable assembled ball joints. This problem can be circumvented by using a collapsible core in the molding process, but that is more expensive and less convenient. Alternatively, the fingers may be formed as a separate assembly from the socket and later mounted on the socket, but this increases the part count, complicates the assembly process, and ultimately increases the cost. Other methods for forming the ball joint call for similar tradeoffs of cost, convenience, and additional stressing of the fingers.